Abstract: A new method to identify the nature of vibrations in molecular crystals as external or internal is proposed. The criterion is that in isotropic mixed crystals of protonated and corresponding perdeuterated compounds, the external motions (phonons) are in the amalgamation limit, while the internal modes, and the "semi-internal" methyl torsions, are usually in the separated band limit. This criterion is supported by experimental isotopic mixed crystal studies of many molecular crystals. Previous criteria, like temperature and isotope shifts, are shown to be less reliable. Anthracene and biphenyl Raman spectra, as well as literature data, are used as illustrations.